Powder metal heat sink for integrated circuit devices

ABSTRACT

Apparatus for use in cooling an integrated circuit structure. The apparatus includes a heat sink having a first portion configured for thermal engagement with an integrated circuit device and a second portion configured for the dissipation of heat into an ambient fluid, such as air. The heat sink is made from a powdered metal which, in one preferred embodiment, includes copper. The heat sink may be formed from the plurality of discrete layers, each layer having a button projecting from one surface, and a depression formed in an opposing surface. The depression is configured to receive a projecting button portion from another layer. In an alternative embodiment the heat sink includes a plurality of plugs projecting from the generally flat surface.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a continuation of application Ser. No. 08/607,235, filed Feb.26, 1996, now abandoned, which is a continuation of Ser. No. 08/166,712,filed Dec. 14, 1993, now issued as U.S. Pat. No. 5,514,327.

FIELD OF THE INVENTION

The present invention relates to electronic systems, integrated circuitand semiconductor devices. More particularly, the present inventionrelates to methods and apparatus for removing heat generated byintegrated circuit structures and devices.

BACKGROUND OF THE INVENTION

Heat is inevitably generated during the operation of integrated circuitdevices. In some instances, the amount of heat generated by the devicecan be sufficient to irreparably damage or even destroy the device.Continuing advances in the number of transistors and other functionalelements contained in a single integrated circuit, and the increasinglyhigh speeds at which integrated circuits now operate, both contribute tothe problem of integrated circuit heat dissipation. This problem hasbecome so severe, for example, that it is alleged one type of advancedmonolithic microprocessor generates sufficient heat in operation tofacilitate cooking. Others require that a powered fan be incorporated onthem to prevent failure of the device.

It is generally well known to provide some sort of heat sink forsemiconductor devices. A variety of methods and devices have beendeveloped for removing at least some heat from an integrated circuitdevice. Typically a unitary heat sink structure has been used. Heatsinks generally include at least a heat-transferring portion proximateto the semiconductor device for extracting heat therefrom, and aheat-dissipating portion remote from the die with a large surface areafor dissipating heat. The heat-dissipating portion is typically formedwith a number of parallel layers, through which air passes to removeheat from the heat sink. Typically, the entire heat sink structure maysimply be disposed on an exterior of a package, such as on the lid of alidded package.

While perhaps suitable for some limited applications, these types ofconventional heat sink devices are generally not commercially practicalfor use except in extreme instances, such as the microprocessordiscussed above. Thus there stills exists a continuing need forpractical methodologies and devices suitable for providing efficientheat dissipation to increasingly complex integrated circuits. It isbelieved the present invention fulfills this need.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an improved heatsink and an improved technique for manufacturing heat sinks.

It is a further object of the invention to provide enhancedmethodologies for removing excess heat from integrated circuit devices.

In accordance with one aspect of the present invention, a novel heatsink structure is formed from a plurality of individual and similarstackable layers. The first, bottom most layer in the stack has agenerally flat top surface with a generally centrally located recessextending into the top surface. The second and subsequent layers in thestack have a top surface similar (e.g., identical, including the recess)to the top surface of the first layer, and have a stepped bottomsurface. A first projection extends from the bottom surface of thelayer, forming a "shoulder" for a second, smaller projection, extendinggenerally centrally from the first projection, forming a button-likeprojection from the "shoulder" or first projection. The button-likeprojection is sized to fit into the recess in the top surface of theprevious layer in an interference fit. Because of the interference fit,the second layer is assembled to the first layer by pressing themtogether under force. This button-like projection of the second layer isthen "captured" by the recess in the top surface of the first layer,forming a secure mechanical assembly of the two layers. Subsequentlayers may be assembled to the first and second layers in much the samemanner. A plurality of layers can be assembled all at once by arrangingthem in a stack and pressing the entire stack together. In one preferredembodiment the bottom surface of the bottom layer is preferablyidentical to the bottom surfaces of the remaining layers.

The recess is located generally in the center of the layer on the topsurface (side). The "shoulder" is formed on the bottom side of the layeras a generally centrally located region of increased thickness. Thebutton-like projection is disposed generally at the center of the layer,opposite the recess, and extending from the "shoulder" region ofincreased thickness. Intimate contact between the top surface of onelayer and the shoulder of a subsequent layer is sufficient ensures agood thermal path from layer-to-layer.

It should be understood that the button element can be provided on thetop surface of the layer and the recess can be provided on the bottomsurface of the layer, but this is not necessarily a preferredconfiguration.

In a preferred embodiment each resulting layer is preferably disc-like,having a circular outline (plan view). However, to better match theoutline of a package (which is typically not circular), the layers canbe formed with polygonal or elliptical outlines. This will result in agreater surface area for each layer, within the "footprint" of thepackage. Similarly, the button-like projection, recess, and "shoulder"can be circular, elliptical, or polygonal. In the event that the outlineof the layer is not circular (e.g., square, rectangular, elliptical)then it has an inherent "orientation". In these and other cases it canbe advantageous to form noncircular shapes for the button-likeprojection and recess to provide "keying", or common orientation betweenlayers, or alternatively providing more than one button-like projection.

According to an aspect of the invention, the button element is slightlylarger (e.g., in diameter) than the recess. For example, the buttonelement may be one-half mil larger than the recess. Additionally, thebutton element may have a taper (draft), for example on the order of 3degrees. This ensures a snug press fit between the assembled layers.Preferably, the recess is at least as deep as the height of thebutton-like projection, to ensure complete mating between adjacentlayers.

According to a feature of the invention, either the recess or the buttonmay be provided with a groove or hole for permitting potentiallyentrapped gases (e.g., air) to escape during the press fit procedure.

According to another feature of the invention, a thermally conductivesubstance, e.g., silicone grease, can be disposed between the assembledlayers (e.g., spread on the shoulder of each layer) to improve thermaltransfer from layer-to-layer. The use of thermal grease, whileadvantageous, if allowed to form a film on either the button or therecess may tend increase the necessity for relief grooves or holes topermit entrapped gases to escape during assembly.

The layers are formed of a thermally-conductive material, such ascopper, a copper alloy, aluminum, or an aluminum alloy. The topmostlayer may have a top surface that is dissimilar from the top layers ofthe remaining layers, but this is not preferred.

The bottom most layer may have a bottom surface that is dissimilar fromthe bottom surfaces of the remaining layers, so that the bottom mostlayer is especially suited to being disposed in close proximity to asemiconductor die or its package, but this is not preferred. Forexample, the bottom surface of the bottom most layer may be formed tomate with an element of the semiconductor package (see, e.g., FIG. 3 ofU.S. Pat. No. 5,175,612, the disclosure of which is hereby incorporatedby reference). Alternatively, the bottom surface of the bottom mostlayer can be identical to the bottom surfaces of the remaining layers,and an adaptor can be interposed between the bottom layers.

According to the invention, any number of layers may be pre-assembledtogether, prior to mounting to a semiconductor package. In this manner,a wide variety of heat sinks can be formed for different applicationsfrom a supply of identical layer elements.

In use, the layers are assembled together by press fit, and the assemblyis mounted directly to a semiconductor die package, such as by gluing(with a thermally-conductive adhesive) to the lid of the package.Alternatively, an "adaptor" may be interposed between the bottom mostlayer and the semiconductor package. Preferably, the layers are pressedtogether prior to assembling the heat sink to a semiconductor assembly.Alternatively, a first layer can be assembled to the semiconductorassembly and remaining layers can be pressed into the first in aseparate operation. This is not preferred, however, as it may subjectthe semiconductor package to excessive forces.

Commonly owned U.S. patent application Ser. No. 08/093,292, filed Jul.15, 1993, provides additional detail regarding suitable heat sinks andmanufacturing methods, and its disclosure is hereby incorporated by thisreference.

In accordance with a another aspect of the present invention, a heatsink suitable for dissipation of heat from integrated circuit devices ismanufactured from a powdered metal. The heat sink may be formed intostackable elements, in accordance with the structural aspect of theinvention described above, or it may be formed into otherconfigurations. The inventor has determined powder metal technology iswell suited to the manufacture of heat sinks. In contrast toconventional fabrication methods, it is not necessary to form a slab ofthermally conductive material and then remove portions of the slab bycostly machining steps to achieve a desired shape. The advantages ofusing powder metallurgy (versus machining solid metal) includeeliminating or minimizing machining and eliminating or minimizing scraplosses. The inventor has further determined that suitable thermaltransfer efficiencies can be achieved in heat sinks made from powderedmetals, despite the typically high porosity and conmitant reducedthermal efficiency of powdered metal artifacts compared to solid metalartifacts formed by conventional machining techniques.

In a preferred embodiment of this aspect of the present invention, a dieis provided having a predetermined shape. Powdered metal may then beintroduced into the die. The powdered metal may be mixed with suitablebinder materials before introducing the mixture into the die. Thebinders may be selected form any binders known for use in the powdermetallurgy art. The powdered metal is then compressed to form a heatsink and may be simultaneously or thereafter sintered at a temperaturesubstantially above room temperature. During the sintering step, thebinders may be removed from the heat sink structure.

The die may have two halves, with the powdered metal deposited into afirst mold half and with the second mold half being used to compress andshape the powdered metal within the first mold half. Preferably, thepressure during compression is selected from pressures ranges normallyused in the industry. The temperature during compression preferably isselected from temperatures commonly used in fabricating structures fromthe specific powder metal selected. The time and temperature ofsintering will also vary with the composition of the powder used, thephysical characteristics of the heat sink, and the quantities and typesof binders, if any, used. The duration of sintering is at least longenough to produce a structure having desired characteristics, such as anappropriate hardness and thermal conductivity.

Preferred powders for use in accordance with the present inventioninclude copper, aluminum, tungsten, titanium and alloys thereof. In onepresently preferred embodiment, the powder is powdered copper, oranother metal or metal alloy having a thermal conductivity at leastequal to that of copper. Copper has excellent thermal conductivitycharacteristics, but is generally regarded as far too expensive for useas a heat sink in connection with cooling integrated circuit devices.This is due at least in part to the tedious nature of conventionalfabrication processes requiring machining the heat sink structure from ablock of solid copper and the associated waste of machined metal.Typically the cost of raw copper stock and conventional machining toform the heat sink structure results in a heat sink for an integratedcircuit device which costs as much or more than all but the mostexpensive integrated circuits. By contrast, through the use of powdermetallurgy, powdered copper and related alloys can be used economically,because waste is substantially avoided and the conventional machiningsteps may be entirely eliminated.

In an alternative embodiment of this aspect of the present invention,the heat sink is initially formed to a first predetermined shape bypowder metallurgy, and is then further shaped by machining. Thiscombined manufacturing technique may be used when the ultimate desiredshape is one which may not readily be produced by compression of powdermetal in a die, but which may be produced by subsequent machining of thecompressed powder metal structure.

Still another aspect of the present invention concerns the formation ofthe novel heat sink structure discussed above from a powdered metal, apowdered metal compound, or a powdered metal alloy. Thus, in this aspectof the present invention the thin layers of the novel heat sinkstructure discussed may be formed of a powdered metal, or may be formedof stamped out metal. It is also possible to form some of the thinlayers from powdered metal and others from stamped metal. When the thinlayers of the heat sink are made from powdered metal, they arepreferably made in accordance with the second aspect of the presentinvention described above.

By using the stackable thin configuration of the novel heat sinkstructure discussed above and powdered metallurgy fabricationtechniques, an inexpensive heat sink may be provided from powderedmetals whereby inexpensively fabricated heat sink layers are pressedtogether to construct the desired number of layers which constitute theentire heat sink structure. By using powdered metallurgy fabricationtechniques to form the heat sink layers, as opposed to the costlyconventional solid metal machining techniques powdered copper and copperrelated alloys may be used to form the heat sink structure withoutproducing a heat sink that is as expensive as the majority of integrateddevices to which it may be applied.

A further aspect of the present invention concerns a novel packagedsemiconductor device, wherein the package includes a heat sink made frompowdered metal and/or formed in a stackable manner. The heat sink may bemanufactured in accordance with any of the methods described herein.

Still a further aspect of the present invention concerns a method ofcooling a semiconductor device by providing the device with a powderedmetal heat sink that may be formed of the novel stackable structuralelements discussed above, and exposing this heat sink structure to acooling fluid, preferably air. The cooling fluid may be forced past theheat sink by mechanical cooling means, such as a fan, or naturalconvection may be used. The mechanical cooling means is preferablyplaced in proximity to the heat sink, and may be mounted directlythereon. This embodiment is most preferably used for semiconductordevices which generate substantial quantities of heat, such as 80486type microprocessors or processors like Intel's Pentium chip.

From the above summary of the invention, it is clear the presentinvention provides both novel and nonobvious heat dissipation structuresand methodologies suitable for use in connection with cooling integratedcircuit devices. Other objects, features and advantages of the inventionwill become apparent in light of the following detailed descriptionthereof.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1a is a cross-sectional view of four stackable layers of a stackedheat sink assembly, according to the invention, prior to assembly;

FIG. 1b is a cross-sectional view of an assembled heat sink assemblyincluding four layers, according to the invention;

FIG. 2 is a perspective view of a doughnut-shaped adapter, according tothe invention;

FIG. 3 is a cross-sectional view of a reducing adapter, according to theinvention;

FIG. 4 is a cross-sectional view of an assembly of a stacked heat sinkarrangement to a glob-top type package, according to the invention;

FIG. 5 is a view of an assembly of a stacked heat sink arrangement to alid of a lidded (e.g., ceramic) semiconductor device package, accordingto the invention;

FIG. 6a is a perspective view of a layer incorporating a gas reliefgroove in the recessed portion thereof, according to the invention;

FIG. 6b is a perspective view of a layer incorporating a gas reliefgroove in the button-like projection portion thereof;

FIG. 6c is a cross-sectional view of a layer incorporating a gas reliefhole, according to the invention;

FIGS. 7a-c are an illustration of one preferred method in accordancewith the invention for fabricating a powdered metal heat sink;

FIG. 8a is a plan view of a heat sink according to the invention; and

FIG. 8b is a side view of the heat sink of FIG. 8a.

FIG. 8c is a side view of an alternative structure for the heat sinkshown in FIG. 8a.

FIG. 9 is a "flip-chip" structure with which the present invention canbe used.

DETAILED DESCRIPTION OF THE INVENTION

In accordance with one aspect of the present invention, a stacked heatsink assembly is formed of a number of individual and stackable layers.A first, bottom most heat sink layer in the stack has a generally flattop surface with a generally centrally located recess extending into thetop surface. The second and subsequent layers in the heat sink stackhave a top surface similar (or, for example, identical, including therecess) to the top surface of the first layer. These subsequent layersfurther include a stepped bottom surface. A first projection preferablyextends from the bottom surface of the layer, forming a "shoulder" for asecond, smaller projection, extending generally centrally from the firstprojection, forming a button-like projection from the "shoulder" orfirst projection. The button-like projection is sized to fit into therecess in the top surface of the previous layer in an interference fit.

FIG. 1a shows such an arrangement 100a of heat sink layers prior toassembly. The arrangement 100a includes four layers 110a, 110b, 110c,and 110d. The top-most layer 110a, representative of the others (110b,c, and d) has a substantially flat top surface 125, with a recess 120formed therein in a generally central location.

A shoulder projection 130 extends from a central region of the bottomsurface of the layer 110a. The shoulder region is somewhat thicker (suchas twice as thick) as the outer layer region. A button-like projection135, smaller (in width) than the shoulder projection 130, extends adistance approximately t/2 from a central portion of the shoulderprojection 130. The button-like projection 135 is preferably locatedimmediately under (opposite) the recess 120. Although not shown in theFigure, edges of the button-like projection 135 and recess 120 arepreferably chamfered, to facilitate inserting the button-like projection135 of one layer (e.g., 110a) into a recess 120a of the next lower layer(e.g., 110b). The four layers 110a, 110b, 110c, and 110d are shownarranged in a stacked vertical configuration with the button-likeprojection of one layer positioned above and extending towards therecess of the next lower layer. The direction of assembly is shown witha series of arrows. The button-like projection 135 is slightly larger(e.g., in diameter) than the recess 120. For example, the button elementmay be one-half mil (0.0005 inches) larger than the recess.Additionally, the button element may have a taper (draft), for exampleof about 3 degrees. This ensures a snug press fit between the assembledlayers. Preferably, the recess is at least as deep as the height of thebutton-like projection, to ensure complete mating between adjacentlayers. In order to assist in fitting the layers together, it ispossible to maintain the layer having the exposed recess at atemperature somewhat higher than the layer having the exposed button tobe inserted into the recess. This causes the opening of the recess toexpand and more readily accept the button.

In another aspect of this embodiment, the assembled layers fit togethercomparatively loosely, and are assembled together prior to sintering themetal. During the sintering process, the assembled layers may be fusedtogether to produce a heat sink with good thermal conductivity betweenthe layers.

The layers may be formed of powdered metal as discussed more fullybelow, but may also be formed of stamped out metal. In either case, thelayers are preferably formed of a thermally-conductive material such asaluminum, copper, or copper/tungsten. Copper is a preferred choice ofmetals.

FIG. 1b shows an assembled stacked heat-sink assembly 100b, formed bypressing the arrangement 100a of FIG. 1a together in the direction shownby the arrows. A press fit is formed between the button-like projectionof each layer and the recess of the next lower layer. The shoulderprojection 130a and button-like projection of the bottom most layer 110dextend from the bottom of the assembly. The shoulder projection (e.g.130) of each layer is in direct contact with a central region of the topsurface of the next lower layer, providing good thermal conductivitytherebetween.

In order to further promote thermal conductivity between the layers ofthe stacked heat sink assembly, it is possible to dispose a small amountof a thermally conductive material, e.g., silicone grease (not shown),between the layers prior to assembly. The thermally conductive material,by increasing thermal contact area and by filling tiny gaps between theshoulder projections and top surfaces of the layers, serves to improvethermal conductivity between the layers.

Although FIGS. 1a and 1b depict an assembly of four layers, any numberof the layers equal to or greater than two can be assembled into asimilar stacked heat sink assembly.

Each layer is preferably disc-like, having a circular outline (lookingdown onto the heat sink). However, the layers can also have polygonal orelliptical outlines.

Each button and recess (and shoulder portion) similarly have a circularoutline (looking down onto the heat sink). However, the button andrecess (and shoulder portion) can also have polygonal or ellipticaloutlines. In the event that the overall shape (outline) of the layer isnot circular (e.g., square, rectangular, elliptical) then it has aninherent "orientation" not found in circles. Accordingly, in such casesit can be advantageous to form non-circular, or otherwise "keyed" shapesfor the button-like projection and recess to provide a common, uniformorientation for the assembled layers. Alternatively, however, the layersmay be provided with two or more buttons and corresponding recessesarranged in an orientable pattern.

The top-most layer can have a top surface that is dissimilar from thetop layers of the remaining layers, but this is not preferred.Similarly, the bottom most layer can have a bottom surface that isdissimilar from the bottom surfaces of the remaining layers, but this isnot preferred. It is preferred that each layer is a replication of eachother layer.

In use, the layers are assembled together by press fitting, and theassembly can be mounted directly to a semiconductor die package, such asby gluing (with a thermally-conductive adhesive) to an exterior surfaceof the package.

Alternatively, an "adaptor" may be interposed between the bottom mostlayer and the semiconductor package. Examples of such adapters are shownin FIGS. 2 and 3.

FIG. 2 is a perspective view of a diameter-increasing, doughnut-shapedadapter 200 used to increase the effective diameter of a roundbutton-like projection of a layer (e.g., of the bottom most layer). Arecess or hole 220, generally centrally located in the adapter 200, issized and shaped to form an interference fit with a mating button-likeprojection (e.g., 135a, FIG. 1b). The outer edge 210 of the adaptereffectively provides a larger diameter button-like projection from alayer to which the adaptor is assembled. This larger diameter projectioncan be used, for example to form an interference fit with a large(larger than the button-like projection of the layer) recess in asemiconductor package to which the stacked heat sink is to be assembled.

Additionally, the doughnut-shaped adapter 200 can be used simply toincrease the effective surface area (footprint) available for adhesionto (and thermal conduction from) a planar surface of a semiconductordevice package, such as to the lid of a lidded package. In this manner(using the adaptor 200), the bottom most layer can have a button ofgreater effective contact area than the remaining layers, although thebottom most layer is identical to each of the remaining layers.

FIG. 3 is a cross-sectional view of a reducing adapter 300, having a toprecess 320 sized to form a press fit (interference fit) with abutton-like projection of a layer, and having a smaller button-likeprojection 310 opposite the recess. This adapter can be used tofacilitate a press fit into a package recess which is smaller than thebutton-like projection of a layer.

Although the adapters 200 and 300 shown and described with respect toFIGS. 2 and 3, respectively are generally assumed to be round in shape,they can also be shaped to accept elliptical or polygonal button-likeprojections, and to provide non-circular button like projections (e.g.,310 FIG. 3) or outer surface shapes (e.g., 210, FIG. 2). In other words,the adaptor 200 may have a circular hole 220 for accepting the button ofthe bottom most layer, and can have a square outline that matches thearea of a lid of a lidded package.

It should be understood that the layers may also be secured to oneanother without using the button-like projections such as button 310 andcorresponding recesses such as recess 320 shown in FIG. 3. For examplethe layers of the heat sink may be bonded together using a thermallyconductive adhesive.

FIGS. 4 and 5 show examples of package assemblies incorporating thestacked heat-sink assemblies described hereinabove.

FIG. 4 is a cross-sectional view of a semiconductor package assembly 400formed on a planar substrate 410. A semiconductor die 420 is attached tothe planar substrate (e.g., with a suitable adhesive). Connections (notshown) are formed between the semiconductor die 420 and conductivetraces (not shown) on the substrate. A dollop 440 of epoxy or otherencapsulant is used to cover the die and its electrical connections. Astacked heat sink assembly 430 (see, e.g., 100b, FIG. 1b) is embedded inthe epoxy dollop 440 such that the button-like projection on its bottomsurface is in close proximity with the semiconductor die 420, therebyproviding means for dissipating heat generated in the operation of thedie 420. In operation the heat sink may be exposed to an ambient fluid.In one preferred embodiment this fluid may be air. This ambient fluidmay also be circulated past the heat sink to enhance heat dissipation.

FIG. 5 is a side view of another semiconductor package assembly 500incorporating a stacked heat sink assembly 530. In this case a package510, such as a ceramic package, with a metallic lid 540 is used. Thestacked heat sink assembly is attached by its button-like projection tothe lid 540 by using a heat conductive adhesive, thereby providing forheat transfer from the package by conduction from the lid 540. Asmentioned above, an adaptor (e.g., 200) can be used to increase thecontact area between the bottom most layer and the lid of the package.

While FIGS. 4 and 5 illustrate use of the heat sink in connection with asemiconductor package, it should be understood that the heat sinkassembly of the invention may be advantageously used to dissipate heatin a wide variety of electronic systems. For example, the heat sink ofthe present invention is also suitable for use in connection with"packageless" semiconductor devices and in connection with multichipmodules. In the latter structure two or more semiconductor dies may beincorporated into a single package or module.

The heat sink of the invention may also be used in connection with socalled "flip-chip" structures in which two or more semiconductor diesare attached to form a single structure. An example of a flip-chipstructure 900 is illustrated in FIG. 9. As shown the structure 900includes a first semiconductor die 902 having an upper surface on whicha second semiconductor die 904 is attached. The area of attachment, thatis the common area of contact between the two semiconductor diesnormally affords electrically interconnective pathways between the twosemiconductor dies.

Accordingly, the heat sink of the invention may be employed inconnection with any existing or contemplated electronic systemsincluding integrated circuit devices and semiconductor devices with orwithout packages, multichip modules, flip-chip devices printed circuitboard structures and other forms of electronic components devices andsystems that can benefit form being cooled.

In the process of press-fitting the layers together, it is possible thatgases (air) will become entrapped in the recess, especially if thermalgrease is used and forms a "sealing" film between the button and therecess. According to a feature of the invention, either the recess orthe button may be provided with a groove or hole for permittingentrapped gases (e.g., air) to escape during the press fit procedure.FIGS. 6a-6c show various exemplary embodiments of gas relief grooves andholes.

FIG. 6a is a perspective view of layer 600a, as viewed generally fromthe top. The top surface 610a of the layer 600a has a recess 620aextending therein, as described hereinabove (e.g., compare 120). Agas-relief groove 640a is formed in the side wall of the recess,providing a path for gas to escape during press fit with a button-likeprojection of another similar layer.

FIG. 6b is a perspective view of another layer 600b, as viewed generallyfrom the bottom. A shoulder projection 630 extends from the bottomsurface 615b of the layer 600b. A button-like projection 635b extendsfrom the shoulder projection 630. A gas-relief groove 640b is formed inthe button-like projection 635b to permit gases to escape duringpress-fit assembly to another similar layer.

FIG. 6c is a cross-sectional view of another layer 600c, illustratingyet another approach to permitting trapped gases to escape duringpress-fit assembly. The layer 600c (compare 430) has a generally flattop surface 610c. Extending into the top surface 610c is a recess 620c.Extending from a bottom surface 615c of the layer 600c is button-likeprojection 635c, located directly under the recess 620c. A gas-reliefhole 640c extends from the center of the button-like projectioncompletely through the layer 600c to the recess 620c on the oppositeside of the layer 600c. The hole 640c is yet another way of permittingtrapped gases to escape during press-fit assembly.

Gas-relief holes may not be necessary in all assemblies, since manypossible layer designs will not trap enough gas or build up enoughpressure to interfere with press-fit assembly or functioning of thestacked heat sink assembly.

According to the invention, any number of layers may be pre-assembledtogether, prior to mounting to a semiconductor package. In this manner,a wide variety of heat sinks can be formed for different applicationsfrom a supply of identical layer elements.

According to an aspect of the invention, the layers are assembled into astack, so that the resulting heat sink has "in" layers, in accordance tothe particular application for which it is intended.

Preferably, the layers are pressed together prior to assembling the heatsink to a semiconductor assembly. Alternatively, a first layer can beassembled to the semiconductor assembly and remaining layers can bepressed into the first in a separate operation. This is not preferred,however, as it may subject the semiconductor package to excessiveforces.

As mentioned above, the inventor regards as another aspect of hisinvention the use of powder metallurgy to form a heat sink structuresuitable for integrated circuit device cooling. Powder metallurgytypically involves the production of a metal powder having suitablecharacteristics and the consolidation of this powdered metal into anartifact by the application of pressure and a simultaneously orsubsequent heating operation. This heating operation, commonly referredto as sintering, may but need not necessarily involve the formation of aliquid phase in the powdered metal material. Satisfactory sintering mayalso be performed below the melting point of all the powdered metalconstituents.

Powdered metallurgy is known in the relevant arts, and powdered metalartifacts are available from a number of companies. One such company isthe Powder Metallurgy Division of St. Marys Carbon Company in St. Marys,Pa. An illustrative example one process for forming powdered metalartifacts is illustrated in FIGS. 7a-7c. While the process illustratedis deemed suitable for fabricating heat sinks intended for coolingintegrated circuit devices, including the novel heat sink structurediscussed above, it should be understood that the inventor regards thepowdered metal manufacturing process shown in FIGS. 7a-7c, and discussedherein as illustrative only. The present invention is not to be limitedto just this illustrative method of fabricating a heat sink frompowdered metal compositions and alloys.

Referring to FIG. 7a there is shown an exemplary molding apparatus 710suitable for the formation of powdered metal artifacts such as the novelheat sink structure discussed above. As illustrated, this apparatus 710includes a mold form 720 defining a bore configured to receive an upperpress 730 and a corresponding lower press 740 in opposing engagement. Tofacilitate formation of an aperture in the resulting powdered metalartifact, the lower press 740 may include an aperture within which acenter pin 750 may be disposed. The upper press 730 may further includean aperture 760 to accommodate the center pin 750.

In operation an appropriate form of shoe 770 is attached to a powderedmetal reservoir (not shown) to provide a quantity of powdered metal 780to the bore of the mold form 720. The shoe 770 is then withdrawn and theopposing upper and lower presses 730 and 740 are forced together withthe powdered metal 780 entrapped between the presses, as furtherillustrated in FIG. 7b. Typically pressures ranging from ten to fiftytons per square inch are used to compact the powdered metalssufficiently to form the desired artifact. Usually larger pressures atthe extreme end of this range are required when larger powdered metalartifacts are formed. After application of pressure to the powderedmetal to form the artifact, the unsintered or green artifact 780a isremoved from the mold form 720, as illustrated in FIG. 7c. The size ofthe compacted artifact 780 typically increases from one-half to one andone-half percent when it is ejected from the mold form 720. Asignificant percentage of force applied to the lower press 740 may thusbe required to eject the artifact 780a from the mold form 720. Asbriefly mentioned above sintering may be performed during or thepowdered metal compaction step or after, at temperatures either above orslightly below the melting point of the powdered metal composition oralloy used. Sintering is typically performed in a controlled atmosphereto prevent oxidation of the powdered metal artifact. Hydrogen ordisassociated ammonia are typically employed for this purpose. It isbelieved that the individuals grains of the powdered metal chemicallybond together due to the diffusion of atoms at the points of contactamong the metal particles, thus forming small contact regions termed"necks". During sintering these contact necks grow larger and developinto metal grain boundaries. Typically the resulting powdered metalartifact is porous, with a density on the order of approximately eightypercent of a comparable solid artifact made from the same material. Thesize of the powdered metal artifact is also effected by the sinteringprocess. Depending upon the type of powdered metal employed, thedimensions of the final artifact may either expand or contract up to twopercent.

As noted above powdered metal artifacts are typically relatively porous.The thermal transfer efficiency of powdered metal artifacts is thususually significantly less than the thermal transfer efficiency ofartifacts made from the same, but solid, metal. Nonetheless, theinventor has determined that heat sinks suitable for integrated circuitdevice cooling can still be fabricated from powdered metals. In actualtesting, the inventor has determined that heat sinks having the flatstacked configuration of the novel heat sink structure discussed aboveand manufactured using powdered metal copper actually providessubstantially greater heat transfer capabilities than comparablestructures machined from solid aluminum. In fact, the inventor's testinghas shown that heat sinks made from powdered metal copper actuallyprovide greater heat dissipation then solid aluminum heat sinks havingeven larger heat dissipating structures. More specifically, tests wereconducted using an integrated circuit device and heat sinks having thenovel stacked layer structure discussed above. Circulating air passedwas passed over an exemplary integrated circuit device at the rate oftwo hundred feet per minute, while a five watt power input was appliedto the device. An 8.5° C. increase in temperature per watt input wasobserved when no heat sink structure was employed on the integratedcircuit device. When a solid aluminum heat sink having the configurationdiscussed above (with four layers) was employed under the sameconditions, the integrated circuit device was found to experience a 5.7°C. increase in temperature per watt of power input to the device. Usingthe solid aluminum heat sink having the configuration discussed above,but with eight layers instead of four, the same testing conditionsresulted in a slight lower 4.7° C. increase in temperature of the testintegrated circuit per watt of power input. A heat sink manufacturedfrom powdered copper and having a same configuration, but with onlythree layers, was found to provide such substantially greater coolingefficiency that the test integrated circuit experienced on average onlya 3.9° C. increase in temperature per watt of power input to the device.Thus, despite the increased porosity and concomitant reduction inthermal transfer efficiency typically characteristic of powdered metalartifacts, the powdered copper metal heat sink was found to stillprovide even better cooling for the test integrated circuit device thenheat sinks having comparable configurations that were made out of solidmetal.

It is important to note that, as stated above, the inventor does notregard the present invention as being limited to the heat sinkconfigurations discussed above. The heat sinks produced from powderedmetal in accordance with the present invention may have the shape asshown in FIGS. 1-6, with any number of multiple layers. Alternatively, aheat sink may be formed having the shape illustrated in FIGS. 8a (planview) and 8b (side view). The heat sink 800 illustrated in FIGS. 8a and8b has a base 801, which is preferably planar, and a plurality of pegs802 or other protrusions having a desired shape. The pegs are used toradiate heat away from the slab, by increasing the overall surface areaof the heat sink. In use, the slab is preferably placed proximate to orcontiguous with a major surface of a semiconductor die, such as thebackside of the die. The protrusions may then be exposed to thesurrounding air to radiate heat from the packaged semiconductor die.

In still a further embodiment of the invention, the heat sink structure800 may also be stacked to form multiple layers as illustrated in FIG.8c. As shown a first heat sink 800a is mated with a second heat sink800b with the base portion 801b of the second heat sink 800b in contactwith the posts 802a of the first heat sink 800a. The heat sinks 800a and800b may be attached to one another using a thermal conductive adhesiveof the form know in the relevant arts. Alternatively, however one ormore of the post 802 may be provided with protrusions 804 and 806 (shownin FIGS. 8a and 8b), the lower surface of the base portion 801b may beprovided with apertures 808 configured to receive these protrusions inpreferably, but not necessarily an interference fit. It should beappreciated however, that other methods of attachment could be utilizedwithout departing from the scope or spirit of the present invention.

By using the techniques of the present invention, an inexpensive heatsink is provided using powder metallurgy technology. In one embodimentinexpensive layers are pressed together to construct the desired numberof layers which constitute the heat sink whole. Using powder metallurgy,copper may be cost-effectively used as opposed to the costly machiningtechniques otherwise necessary to form heat sinks from raw metal stock.Normally the cost of raw copper stock and the amount of material lostthrough machining to form a suitable heat sink structure precludes usingsolid copper metal as a heat sink. Using powder metallurgy to form theheat sink structure, however, makes the application of powder copperpractical for fabrication of heat sinks suitable for use in coolingintegrated circuit devices. The heat sink of the present invention thusprovides a low-cost, high heat dissipation heat sink suitable for usewith a variety of electronic applications, including heat dissipation ofintegrated circuit devices, semiconductor devices, multichip modules,printed circuit structures and other forms of electronic components,devices and systems that can benefit from being cooled.

It will, of course, be understood that various modifications andadditions can be made to the preferred embodiments of the method andapparatus of the present invention discussed above without departingfrom the scope or spirit of this invention. Accordingly, the scope ofthe present invention can not be limited by the particular embodimentsdiscussed above, but should be defined only by the claims set forthbelow and equivalence thereof.

What is claimed is:
 1. A heat sink assembly, comprising:a powdered metalfirst heat sink unit including a first central body and a firstcantilevered fin extending out from said first central body; and apowdered metal second heat sink unit including a second central body anda second cantilevered fin extending out from said second central body;said first and second central bodies being press fit together such thatsaid first and second cantilevered fins are spaced parallel to oneanother and cantilevered out from respective said central bodies.
 2. Theheat sink assembly of claim 1 further comprising a semiconductor packageto which said second heat sink unit is adhesively secured.
 3. The heatsink assembly of claim 1 further comprising a semiconductor package andepoxy securing said second heat sink unit to said package.
 4. The heatsink assembly of claim 1 wherein said powdered metal of said first andsecond heat sink units is powdered copper.
 5. The heat sink assembly ofclaim 1 wherein said powdered metal of said first and second heat sinkunits comprises a powdered metal compound.
 6. The heat sink assembly ofclaim 1 wherein said powdered metal of said first and second heat sinkunits is selected from the group of copper, aluminum, tungsten, titaniumand alloys thereof.
 7. The heat sink assembly of claim 1 wherein saidpowdered metal is a metal or a metal alloy having a thermal conductivityat least equal to that of copper.
 8. The heat sink assembly of claim 1further comprising an adapter interposed between said second heat sinkunit and a semiconductor die package.
 9. The heat sink assembly of claim1 wherein said first and second heat sink units when press fit togetherhave a flat stacked configuration.
 10. The heat sink assembly of claim 1wherein one of said first and second heat sink units has a button-likeprojection and the other has a recess, wherein said button-likeprojection is fit into said recess with an interference fit when saidfirst and second heat sink units are press fit together.
 11. The heatsink assembly of claim 10 wherein said first or second heat sink unithaving said button-like projection has an abutment shoulder which spacessaid first and second heat sink units apart when said first and secondheat sink units are press fit together.
 12. The heat sink assembly ofclaim 11 wherein said shoulder is in intimate thermal conducting contactwith the other of said first or second heat sink unit.
 13. A heat sinkassembly, comprising:a heat sink first layer formed by compressing andsintering powdered metal and binder to form a predetermined shape; aheat sink second layer formed by compressing and sintering powderedmetal and binder to form a predetermined shape; wherein said first andsecond layers are press fit together in parallel orientation; and anadapter interposed between said second layer and a semiconductor diepackage.
 14. The heat sink assembly of claim 13 wherein said adapter isformed separately from said first and second heat sink layers.
 15. Theheat sink assembly of claim 13 wherein said adapter is adiameter-increasing, doughnut-shaped adapter having an opening generallycentrally located in said adapter and an outer edge, said second heatsink layer has a button-like projection, said button-like projectionforms an interference fit with said opening, and said outer edge of saidadapter forms an interference fit with a recess in said semiconductordie package.
 16. The heat sink assembly of claim 13 wherein said adapteris a reducing adapter having a top recess and a button-like projectionopposite said recess, said second layer has a button-like projection,said top recess of said reducing adapter sized to form a press fit withsaid button-like projection of said second layer, said button-likeprojection of said reducing adapter having an outside diameter smallerthan an outside diameter of said button-like projection of said secondlayer, and said button-like projection of said reducing adapter sized toform a press fit with a recess of said semiconductor die package. 17.The heat sink assembly of claim 13, wherein said first and second layerscomprise compressed and sintered powdered metal.
 18. The heat sinkassembly of claim 13 wherein said first and second layers havesubstantially identical shapes.
 19. The heat sink assembly of claim 13wherein said first and second layers are interchangeable.
 20. A heatsink assembly, comprising:a heat sink first layer formed by compressingand sintering powdered metal and binder to form a predetermined shape;and a heat sink second layer formed by compressing and sinteringpowdered metal and binder to form a predetermined shape; wherein saidfirst and second layers are press fit together in parallel orientation;wherein one of said first and second layers has a button-like projectionand the other has a recess; wherein said button-like projection is fitinto said recess with an interference fit when said first and secondlayers are press fit together; and wherein said first or second layerhaving said button-like projection has an abutment shoulder which spacessaid first and second layers apart when said first and second layers arepress fit together.
 21. The heat sink assembly of claim 20 wherein saidshoulder is in intimate thermal conducting contact with the other ofsaid first or second layers.
 22. The heat sink assembly of claim 20wherein a first heat sink fin projects from said abutment shoulder, andsaid abutment shoulder has a width larger than said button-likeprojection.
 23. The heat sink assembly of claim 22 further comprising anadapter interposed between said second layer and a semiconductor diepackage.
 24. The heat sink assembly of claim 20 wherein said first layerincludes a first central body and a first fin extending out from saidfirst central body, said second layer includes a second central body anda second fin extending out from said second central body, and said firstand second layers are press fit together such that said first and secondfins extend out from said first and second central bodies, respectively,in spaced parallel arrangement so as to define a continuousuninterrupted cooling gas layer around said first and second centralbodies and extends from said first and second central bodies to outeredges of said first and second fins and in between said first and secondfins and above and below said first and second fins.
 25. The heat sinkassembly of claim 20 wherein one of said first and second layers has abutton-like projection extending from a surface that is opposite to saidsurface in which a recess is formed, and an abutment shoulder betweensaid button-like projection and said recess.
 26. The heat sink assemblyof claim 20 wherein said first and second layers comprise compressed andsintered powdered metal.
 27. The heat sink assembly of claim 20 whereinsaid first and second layers are interchangeable.